How to Grow Wallflowers in Australia

Wallflower (Erysimum cheiri, formerly Cheiranthus cheiri) belongs to the cabbage family, Brassicaceae. It is grown for spikes of sweetly scented flowers from late winter into spring. It is a short-lived perennial that performs best as a biennial. Sown in summer or autumn, it grows leafy through the cool months, flowers, and is then replaced. The usual mistake is sowing too late for the plants to size up before winter.

Three mistakes that matter

1. Sowing too late. Plants need about 24 to 26 weeks from sowing to flower, most of it as leafy growth. Seed sown at the tail of autumn gives small plants and a thin display. Sow early in your zone's window under When to plant.
2. Wet, heavy soil. Wallflowers hate wet feet, and roots sitting in soggy winter soil can rot before a single spike opens. Grow them in free-draining ground, a raised bed or a pot, as set out in Site, soil and setup.
3. Planting after cabbages. Clubroot, a soil-borne disease of the cabbage family, attacks wallflowers too, and its spores can survive in soil for up to 20 years. Choose a bed that has grown no cabbage-family crop for a few years.

When to plant

Seed comes up in 10 to 21 days at a soil temperature of 18 to 21°C. The warm soil of late summer and early autumn suits it, so sow early in the window.

At the hot start of a window, raise seedlings in trays in light shade and plant them out as the nights cool.

Site, soil and setup

Give wallflowers full sun and free-draining soil that is neutral to alkaline. Moderate fertility suits them: plants in very rich soil tend to die out. In the hottest inland and northern gardens, shade from the afternoon sun can help. A quick soil pH test tells you whether the bed needs lime.

Container growing

Both varieties reach about 40 cm, a size that sits well in a pot. Allow at least 10 L of potting mix per plant; a 30 cm pot holds about 13.5 L and needs drainage holes. Bagged potting mix can carry Legionella bacteria, so wet it before opening it up, wear a mask and gloves, and wash your hands afterwards.

• Watering: water when the mix is dry a few centimetres down, then let the excess drain away.
• Feeding: from the first buds, water on fish and seaweed blend at 25 mL per 9 L every two weeks.
• Position: full sun, out of strong wind, which can push the flower spikes sideways.

Sowing Seeds

Wallflowers do best raised in trays or small pots and planted out 4 to 6 weeks later. Seed can also go straight into the bed.

Starting in trays

• Sow 3 to 6 mm deep in fresh seed raising mix, or two seeds to a 7.5 cm pot, keeping the stronger seedling.
• Keep the mix moist but never wet; seedlings show in 10 to 21 days.
• Move each seedling into its own 10 cm pot once it is big enough to handle.
• Harden off the young plants (give them a little more time outdoors each day over a week or two) before planting out.
• Plant out at 5 to 10 cm tall, 20 to 30 cm apart, and pinch out the growing tip for a bushier plant.

Sowing direct

• Rake the bed fine and sow along shallow drills, 3 to 6 mm deep.
• Water with a fine rose and keep the surface damp until seedlings appear.
• Thin in stages to a final 20 to 30 cm apart.

The two varieties share one method and one timetable from sowing to picking, so choose on colour alone.

Care by growth stage

Wallflowers want little feeding, and compost dug in at planting carries most plants. Soil that drains through winter and stays evenly moist at flowering matters more than any fertiliser.

On thin, sandy soil where plants stall, scratch in a controlled release all-purpose granular (NPK around 14-5-10) at the label rate.

Deadheading

Spent spikes left on the plant turn to seed pods. Once the weather warms, a wallflower can stop flowering and run to seed quickly.

• Fading spikes: snip each one off just above a set of leaves.
• Warm spells: check every few days, because heat speeds seed setting.
• Self-sown seedlings: leave a spike or two to ripen if you want them, knowing they may not match the parent's colour.
• End of flowering: pull out the spent plants and compost them, as they turn leggy in a second year.

Companion planting

Wallflowers bloom with the spring bulbs and cool-season annuals, which makes them easy partners in a bed or pot.

Grows well with

• Tulips: plant the bulbs among young wallflowers in autumn, and both flower together in spring.
• Forget-me-nots: another spring flower that sits well with wallflowers in a bedding scheme.
• Violas: sown in the same late summer and autumn months, they fill the front of the bed.
• Vegetable beds: bees, hoverflies and ladybirds visit the flowers, bringing beneficial insects close to the crops.

Keep apart

• Cabbage, broccoli and kale: all clubroot hosts, so sharing or following them builds the disease in the soil.
• Stock: a cabbage-family flower and a clubroot host too, so rotate it with wallflowers rather than after them.

Harvesting Blooms

Pick stems as the flowers on each spike open, from late winter into spring.

• Cut with sharp secateurs just above a set of leaves.
• Sear the cut ends in boiling water for 10 to 15 seconds, then stand the stems in cold water.
• Add a splash of clear vinegar to the vase water.
• Keep picking through the season, as every cut stem doubles as deadheading.

In a vase, wallflowers usually last five to seven days, and the flowers also dry well for potpourri. Keep them for the vase and not the kitchen: the plant contains cheiranthin and is said to be poisonous in large quantities.

Common problems

Seed fails to come up. Seed buried deeper than 6 mm, mix left to dry out, or soil well below 18°C usually explains it. Read more

Seedlings topple overnight. Damping off (a rot of young seedling stems, mostly from Pythium species and Rhizoctonia solani) thrives in wet mix and crowded trays. Read more

Plants rot at the base in winter. Waterlogged roots are the cause; the second of the Three mistakes that matter gives the fix.

Few flowers on small plants. Late sowing or very rich soil is the likely cause; see Three mistakes that matter and Site, soil and setup.

Flowering ends early. Warm weather makes plants set seed fast, so deadhead as set out in Deadheading.

Pests and diseases

Wallflowers share the soil and leaf diseases of their cabbage relatives, and molluscs go for the young plants.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They chew holes in seedlings and fresh transplants after dark and leave shiny trails. Collect them by torchlight about two hours after sunset, and sink beer traps nearby. Read more

Downy mildew (Hyaloperonospora species). Pale yellow, angular patches appear on top of the leaves with greyish fuzz beneath, in cool, wet, humid weather. Pick off affected leaves, water at soil level and give plants room for air to move. Read more

Clubroot (Plasmodiophora brassicae). Leaves turn yellow and wilt even in moist soil, and the roots swell into knobbly galls. Dig out and remove affected plants, roots and all, then rotate as the third of the Three mistakes that matter describes.

Prevention first

• Give plants the full 20 to 30 cm spacing so leaves dry quickly after rain.
• Water at the roots in the morning, keeping the leaves dry.
• Start every sowing in fresh seed raising mix and a clean tray.
• Check leaf undersides and the base of young plants weekly in cool, wet weather.

Tips

1. Plant in drifts or along the edge of a path, where the scent reaches you as you pass.
2. Set a pot of wallflowers under a window or beside a door, an old trick for enjoying the fragrance indoors.
3. Stock (Matthiola) is a different plant with its own guide, and perennial shrubby wallflowers such as 'Bowles's Mauve' grow from cuttings, not from this seed.
4. Wallflowers can self-seed, so check with your local council about weed potential if you garden near bushland.